This invention relates to a process for the manufacture of a moldable cellulosic fibre-inorganic fibre filled polymer composition and where after injection or compression or compression injection molded into composite product of the same composition with improved properties, preferably tensile strength not less than 75 MPa and flexural strength not less than 125 MPa. The said process consisting of two stages: melt blending of the cellulosic fibre with polymer in the presence of surface active agents under high temperature and shear which results in dispersion of the cellulosic fibres in the polymer, further consolidation and blending with inorganic fibres to moldable thermoplastic composition. The present invention also relates to said polymer composite product manufactured by the said process and to the use of the product within structural applications of automotive, aerospace, furniture and other industries.
Addition of fillers and reinforcement materials to plastics is the typical method used to improve the performance properties, such as tensile strength, modulus values and impact resistance etc. The most widely used reinforcement materials for both thermoplastics and thermosets include inorganic fillers such as glass, talc, and calcium carbonate. Although they impart high strength, dimensional stability and heat resistance to the plastic composite, these fillers are costly, abrade process equipment, and increase the density of the products. As an alternative to these fillers, natural fibres or lignocellulosic fibres have been evaluated as reinforcements for plastics and there is an increasing worldwide interest in the development of natural fibre reinforced thermoplastic/thermoset composite. The interest is driven by the advantages of these natural fibres in contrast to inorganic fillers and includes:                plant fibres have relatively low densities compared to inorganic fillers        plant fibres result in reduced wear on the processing equipment        plant fibres have the advantages of health and environmental issues        plant fibres are renewable resources and their availability is more or less unlimited        composites reinforced by plant fibres are CO2 neutral        plant fibres composites are recyclable and are easy to dispose        complete biodegradable composite product can be made from plant fibres if used in combination with biopolymers.        
However, poor dispersion of fibres in the plastic matrix, lack of interfacial interaction between the fibres and the plastic matrix are still pertaining as the challenges in the development of plant fibre reinforced composite product with improved properties. Poor dispersion, being resulted from the strong hydrogen bonds between the cellulosic fibres and lack of compatibility arises from the very different nature of the hydrophilic surfaces of plant fibres and the hydrophobic nature of the polymeric surfaces. It would be necessary to develop a process for manufacturing high performance composite product with discontinuous cellulosic fibres by properly dispersing the fibres in the plastic matrix. Extensive research work has been done in this topic and several methods have been developed to improve the compatibility between the plant fibres and plastics and also to improve wettability of the fibres with that of the plastic matrix to enhance fibre dispersion.
Use of functionalised polymers and grafting of cellulosic fibres with silane for improving dispersion and adhesion between fibre and matrix have been disclosed by Woodhams in U.S. Pat. No. 4,442,243 (1984) and Besahay in U.S. Pat. No. 4,717,7421 (1988) respectively. Raj et. al in U.S. Pat. No. 5,120,776 (1992) teaches a process for chemical treatment of discontinuous cellulosic fibres with maleic anhydride to improve bonding and dispersability of the fibres in the polymer matrix.
Horn disclosed, in U.S. Pat. No. 5,288,772 (1994), the use of pre-treated high moisture cellulosic materials for making composites. Kubat et. al disclosed, in U.S. Pat. No. 4,559,376 (1985), a pre-hydrolytic treatment or a chemically degrading treatment of the fibre prior to compounding of the cellulosic fibre to improve the dispersion of the said fibres in the thermoplastic matrix. A hydrolytic treatment of the fibres at a temperature of 160-200 degree centigrade using water as the softening agent has been claimed by Pott et. al in a Canadian patent, Patent No. CA 2235531 (1997). Sears et. al disclosed a reinforced composite material with improved properties containing cellulosic pulp fibres dispersed in a high melting thermoplastic matrix, preferably nylon [U.S. Pat. No. 6,270,883 (2001) and EP No. 1121244 (2001)]. Rana et. al, in two journal articles (Composite Science and Technology, 63, 2003, 801-806 and Journal of Applied Polymer Science, 69, 1998, 329-338) published a single stage processing of jute fibre reinforced thermoplastic composites by different combinations of compatibilizers and impact modifiers.
Performance of a discontinuous fibre filled composite is also depends on fibre length. For example, longer discontinuous fibres have the capacity to withstand greater stress and hence have greater tensile properties than shorter fibres of similar nature, as longer fibres can absorb more stress prior to failure than a shorter fibre. Jacobsen disclosed in the U.S. Pat. No. 6,610,232 (2003) the use of long discontinuous lignocellulosic fibres for thermoplastic composites.
Medoff and Lagace in the U.S. Pat. No. 6,207,719 (2001) disclosed manufacturing of texturised cellulosic and lignocellulosic materials. Further they give example to use texturised fibres for making composite using conventional equipment such as a roll mill.
Although prior art show processing of thermoplastic composites containing lignocellulosic fillers with different combinations of thermoplastics, cellulosic fillers including texturised cellulose fillers and coupling agents, and fibre treatments, the particular feature of the present invention are absent in the prior art. The prior art is deficient in producing high strength performance cellulosic filled thermoplastic composite materials, which is overcome by the present invention that leads to development of high strength composite materials from inexpensive thermoplastics and lignocellulosic fibres.